• Andre Brown

Hypocycloid "Square" Engine

Whilst a student at the University of Bath, where I undertook an MEng in Automotive Engineering in 2002, I conceived of a novel engine design using cams and a linear sliding crank arm that held the piston almost stationary at top dead centre. It was my belief that this would ensure combustion at constant volume, the ideal process, and that this would lead to an increase in engine efficiency vs traditional piston engines.

My thinking was based on the fact that max cylinder pressure occurs somewhere near 10 degrees after top dead center TDC. max pressure = max force. If the max force was converted to torque via such a small crank angle, surely delaying max pressure to a later crank angle (say 45 degrees) would increase engine torque for a given cylinder pressure. Power extracted was a integral function of torque transmitted over crank revolutions, so delayed pressure has a negative impact in this regard, however a one dimensional combustion model in excel suggested BSFC and IMEP could be increased.

I later revised this design when I learned of the beauty of hypocycloids, a fascination with maths that I still hold today. It was clear to me that a geared cycloid could control piston motion in a reliable way. seems like this wasn't a new idea - Matthew Murray invented a similar engine way back in 1802 - one of the earliest steam engines. In this design, the small gear is half the diameter of the larger gear, causing the piston to travel perfectly straight up and down. by changing the ratio of the diameters, different piston paths could be obtained.

My new design allows the piston to be perfectly stationary at TDC by choosing a ratio that gives an arc to the cycloid shape that matches the con-rod length (as you can see in the youtube videos below). The mechanical embodiment is simple and is realistic considering the operating conditions inside an engine - just consisting of gears and bearings - it should be possible to build one and make it work. I knew that a higher level of simulation was needed before committing to the cost of a prototype, so I enlisted the help of a talented student, Michael Rhodes (Cranfield University 2013) who wrote his research thesis on simulation of this engine using AVL Boost. Michael is now an engine designer at Ilmor.

His conclusions are that there is promise here to make a more fuel efficient engine, however, more investigation is needed into turbo charging and variable valve timing to solve the problems with needing to fill the cylinders in a shorter time due to the increased dwell, and the geometric issues with bottom dead centre that would cause pumping losses with conventionally timed exhaust valves. Both of these I believe are solvable, however the inherently higher piston accelerations (due to increased dwell time for a given rpm), suggest that the design may be more suitable for lower revving and constant speed engines like ships, trains, or generators, and maybe less applicable to cars.

Finally, I am releasing this work as open source, under the open source hardware license (OSHW). I believe that the future of personal automotive transport is electric, however I would be delighted to work with any people/companies who feel this work could help make their products more efficient where electric motors are not yet practical. The youtube videos below state confidential, however I made these a while back, and my views on the value of patents have changed since! a subject for a later blog post.

Below is my 3D CAD model of the engine, plus Michael's comprehensive research thesis.

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